SECONDARY TREATMENT OF WATER BY FILTERATION

FILTRATION

The most important stage in water treatment is filtration. In filtration the water is passed through he thick layer of sand. Through filtration following effects are seen:

• Chemical properties of water are changed.
• Suspended and colloidal impurities in finely divided state are removed. 
• The number of bacteria are also reduced.

The filtration is based on four actions:

1) Mechanical Staining

The suspended particles that can not pass through the pores of sand are arrested and removed through the  action of mechanical straining.

2) Sedimentation

The spaces in the sand particles act as the small sedimentation tanks. The impurities are arrested in voids of sand and these adhere to the particles of sand due to 2 reasons:

• due to physical attraction between two particles of matter
• due to presence of gelatinous film developed on sand grains by previous bacteria.

3) Biological Metabolism

The growth and life processes in living cells is called as biological metabolism. The action of filter is based on biological metabolism process. When bacteria are held in voids of sand, a zoological jelly is formed around. The film consist of large colonies of bacteria. Bacteria feed on organic impurities in water. They convert impurities to harmless compounds by biochemical reactions.

4) Electrolytic Changes

The action of filter can also be explained by ionic theory. It describes that when two opposite charges come close in contact to each other the charges are neutralized and new chemical substances are made. It is seen that some of the sand grain filters are charged with the electricity of some polarity. When particles of suspended and dissolved matter containing electricity of opposite polarity come in contact with sand grains, they neutralize each other and results in alteration of chemical properties of water. 

FILTER SAND

The sand used in filter is free from clay, loam, vegetable matter and organic impurities. It should be uniform in nature and size. The classification of filter sand depends upon effective size and uniformity coefficient. The effective size of sand depicts the size of sieve in mm through which 10 % of sample by weight is passed. The uniformity coefficient is the ratio of sieve size in mm through which 60 % of the sample of sand by weight will pass to the effective size of sand.

CLASSIFICATION OF FILTERS

The filters are classified into two categories:

• Slow Sand Filters
• Rapid Sand Filter

Rapid sand filters are further categorized into two sub classes:

• Gravity type rapid filters
• Pressure type rapid filters

SLOW SAND FILTERS

The purpose of slow sand filters is to pass the water slowly through the layer of sand that is placed above the base material and thus purification process is aimed to improve the biological, chemical and physical properties of water. These are best suitable for rural areas in developing countries. because of its simple and maintenance procedures. It pure water at low cost. 

ESSENTIAL PARTS 

A slow sand filter consist of following parts:

• Enclosure tank
• Under drainage system
• Base material
• Filter media of sand
• Appurtenances

The rate of filtration from slow sand filters varies from 100 to 200 liters / hour/ meter square of filter area. 

EFFICIENCY OF SLOW SAND FILTERS

• They remove about 98 to 99 % of bacterial load from raw water.
• They may remove 20 to 25 % color of raw water.
• They can remove the turbidity to an extent of 50 ppm. 

RAPID SAND FILTERS

The main disadvantage of slow sand filters is that it needs considerable space for its installation. This makes it uneconomical for the places where the land values are high. This led the engineers the need to increase the rate of filtration that could be increased by two ways:

• by increasing the size of sand so that the friction of water passing through the filter media is minimized.
• by allowing the water to pass under pressure though the filter media

The first one is achieved by gravity type rapid sand filter and second is achieved by pressure rapid filters. 

The parts of rapid sand filters are same as the slow sand filters.